1. Field of the Invention
This invention relates to controlling automatic transmissions.
2. Prior Art
Various hydraulic transmission systems are known. For example, it is known to have various hydraulic logic systems controlling hydraulic valves governing transmission shift pressure and a transmission input torque capacity pressure. In turn, hydraulic force from two hydraulic lines carrying these two pressures is applied to a shift valve to cause shifting between gears. Also known are electronically controlled transmissions wherein electrical logic signals are used to activate a solenoid which then permits hydraulic shifting of the gears. However, such systems are not readily compatible with one another and each requires unique specialized apparatus not required by the other. It would be desirable to achieve an improved electronic transmission control without the necessity for a complete modification of the hydraulic transmission apparatus. These are some of the problems this invention overcomes.
Some known transmission systems have shifts between transmission gear ratios caused by a shift valve responsive to a ratio between a TV (throttle valve) pressure indicating driver demand caused by gas pedal depression and a governor pressure indicative of vehicle speed. Various ratios of TV pressure/governor pressure cause successive gearshifts between first and second gear, second and third gear, and third and fourth gear. To maintain, for example, second gear, the desired ratio of TV pressure/governor pressure would be someplace between the pressure ratio value causing the shift from second to first gear and the value causing the shift from second to third gear. To get the governor pressure corresponding to a particular pressure ratio, the pressure ratio is multiplied by a function of the TV pressure. If the actual gear is different from the desired gear, the transmission is checked for switching between transmission gear ratios. If no gear change occurs, the governor pressure is ramped until a gear change occurs.
The use of a solenoid valve for controlling the pressure of a hydraulic medium is further described in U.S. Pat. No. 3,727,487 issued Apr. 17, 1973, to Forster et al, entitled "Pressure Control Installation for Hydraulically Shifted Automatic Transmissions, Especially for Motor Vehicles," the disclosure of which is incorporated herein by reference. The following patents are generally related to the use of a solenoid in an electronic control system to provide signals to hydraulic shift valves causing transmission gearshifts: U.S. Pat. Nos. 4,044,634; 4,082,013; 4,107,776; 4,252,148; 4,261,229; 4,308,764; 4,314,340; 4,335,428; 4,338,666; 4,351,206; 4,354,236; 4,367,528; 4,380,048; 4,388,845; 4,393,467. However, these patents typically teach the use of a plurality of solenoids in combination with a plurality of hydraulic shift valves to shift the transmission gears so that there is one or more shift valve per gear ratio. It would be desirable to achieve a simpler transmission control. For example, in addition to the one solenoid typically used in all hydraulically controlled automatic transmissions for line pressure control i.e., control of the transmission input torque capacity pressure, it would be desirable to use only one additional solenoid for all shift related control regardless of the number of available gear ratios. Further, none of these patents teach calculating a throttle valve/governor pressure ratio for use in controlling the transmission.